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Tuesday, January 24, 2012

Exactly one year ago, a first post (an introductory one) was posted on Legoism. I'm glad to see that the number of its page views has been continuously climbing month by month though it's, of course, still light-years away from some more established Technic blogs (many of which you can find in the Links section). I will, as the picture above suggests, try to improve it in the future.

A few numbers: the blog is now close to reaching the 25,000th page view (should be reached in about a week or so). The post with by far the most views, over 5000, is a review of 8070 Super Car ― it has roughly as much as the next four: all being reviews, from 5893, over 8053 and 8258, to 8051. It's a bit depressing that the next two posts are actually documenting two failures: the horrible Technic Supercar, and the unsuccessful NXT plotter, and only then the TGB Car. That's something I obviously need to work on. Interestingly, lots of visitors actually come to this site via image search.

Thank you everyone for visiting and reading Legoism so far ― I'll try to come up with lots of exciting stuff in the future!

Sunday, January 22, 2012

After a few previous posts regarding differential drive chassis and its self-steering developments, it might be useful, before we move on, to offer a table comparing all these drive configurations ― their pros and cons. Here we'll restrict the table to 4-wheeled types, though general ideas also apply to those with three or more pairs of wheels. Click on the table to zoom it for easier viewing.

Sunday, January 15, 2012

Experienced builders are well acquainted with many ways to reduce several functions to a single control axle, but the concept is important enough to present it to Technic beginners. Namely, typical approach, which works just fine for majority of MOCs, uses one control axle (or a motor) for each function. However, mobile and remotely controlled models often set strict limits on the number of electric components one can use, and in such situations it is necessary to go for alternate solutions - among which this differential-ratchet-gearbox system is just one of possibilities.

A careful look at the photos should reveal at least a little bit of how this system works ― but let's describe it component by component anyway. The input axle, in this case attached directly to a motor (A) is directly linked to a differential (B) master gear. Each of its two outputs leads to ratchets (C) that limit the rotation in different directions. This twin-ratchet setup ensures that the different directions the input axle is rotated are split among two exiting axles; this can itself be a useful component.

One output axle drives the input of a simple gearbox (D) which, in this case, has three output gears, though designs with four or more shouldn't present a problem. However, there is only one sliding intermediate gear, ensuring that the input rotation can be linked to only one output axle (E) simultaneously. Of course, the other axle from the differential controls the movement of this gear. In this case, it is done with a simple worm gear, spur gear and an off-center set control beam (F).

In other words, driving the motor in one direction changes the active output axle of the gearbox in the 1-2-3-2-1 order. Running it in the other direction transfers its motion to the selected active axle. Of course, the entire system can be built more compact in practice ― this display version was designed to avoid the components being obfuscated.

There are drawbacks one needs to be aware of, of course ― most importantly, the output axles rotate in one direction only (though, if they control a linear motion, the aforementioned off-center set beam connections can help). This system also allows the unlinked axles to rotate freely in one direction, and introduces some friction (thus reducing available torque). It's too large to cram comfortably in smaller models, and finally, it's somewhat noisy. But in certain situations, the benefit of controlling everything with just a single input axle, i.e. one motor, easily overshadows these remarks.

Venturing a little further into theory, it is conceivable to attach each of the output axles to off-center linkages that control Power Function switches connected to the motors attached to further such systems, squaring the number of available output functions (output axles). However, such system would be incredibly cumbersome to use.

Friday, January 6, 2012

Some may remember a very simple self-steering chassis I've tested last February. It was about time to advance it, so here's a new self-steering chassis concept, Mark II.

Let's just quickly remind ourselves what the self-steering chassis is about, and when would one prefer it over typical designs. Namely, if a planned vehicle is limited to using only two motors altogether (due to weight, power requirements, size, simplicity or any other reason), a conventional approach would use one to provide drive, and the other to steer. However, for some particular models it might be troublesome to use only half the available power for providing drive.

An alternative is to have both motors provide drive, one for each wheel. In such configuration, steering is done by having the two motors (and of course, their respective wheels) turn at different speeds, thus producing sufficient tangential force for the chassis to turn. This type of drive, often known as a differential drive, is used by tanks, for example. However, if the chassis is wheeled, it will turn too ― at least if the front wheels have sufficient caster to readily follow it, what was the point of the aforementioned Mark I chassis. But it still suffers from poor reliability unless perfectly balanced and having minimal friction, both of which are sometimes unavoidable. Therefore, an obvious direction for improvement is to introduce a steering control that follows the drive wheels' behaviour.

Mark II concept attempts to solve this problem mechanically. To explain its functioning, let's split it down to a few main components.

Each of the rear wheels' axles (A) is linked to a differential several studs ahead (B). As you have perhaps noticed, there are three gears (C) on one, and four at the other side, resulting in one of the axles spinning in reverse. The differential thus works opposite from its typical application on the half-axle: it actually turns only when the rear wheels aren't turning at the same speeds (i.e. they tend to steer), and its rotation speed is proportional to the difference between the rear wheels' speeds. But as long as the rear wheels turn simultaneously, it remains stationary.

In other words, its turning speed is proportional to the amount of steering that needs to be done by the front wheels ― of course, also obeying the direction. But since it's the speed and not the absolute displacement we're interested in, this differential cannot be directly linked to the steering pinion. Instead, we need a simple sort of a mechanical tachometer, and that is approximately what the front differential (D) does. It has got a very light brake (E) at its main axis, and its second axle leads to a usual steering rack&pinion mechanism (F).

The point is that the brake and the rack&pinion work in balance: the more torque and speed arrive from the rear differential, the more will they be met by the increasing resistance of the brake, thus directing more towards the steering. The steering, of course, imparts its own resistance depending on how sharp turn it's attempting to make, and yet more if it's equipped with a recentering system (G). Exact steering would need a very precise balance between these variables, but in practice it needs no more than a nudge in the correct direction, and then the friction to the ground keeps it in correct geometry. In other words ― it works.

When there is no need to steer anymore (the rear wheels begin turning at the same speed), the rear differential will stop turning, and it will be easier for the front wheels to recenter themselves thanks to a bit of caster. Unless the weight on the front axle is huge and the tyres grippy, this force will be very light ― and that is the main reason why the pinion couldn't be linked to the rear differential through a standard clutch gear: its resistance would almost always be too high, and prevent the front wheels from recentering. Very heavy models may still get away with it, though ― but they rarely have limits on the number of motors anyway. The better but more complex solution is to introduce a recentering mechanism and adjust its force carefully according to the rest of the system, in which case the caster is not necessary.

There are still some particular situations in which Mark II works poorly, such as turning in place (with rear wheels turning in opposite directions), turning slowly in sharp curves, and driving backwards if the steering is centered only by caster (without recentering mechanisms). However, in typical circumstances it works all right.